The present invention relates to apparatus for improving the low speed and cold weather performance of two-cycle engines and, more particularly, to apparatus which utilizes the engine exhaust to heat induction air to the carburetor to prevent fuel puddling in the crankcase and carburetor icing, and to improve idle speed carburetor calibration.
Two-cycle marine engines, of the type commonly used for outboard motors, utilize the crankcase as part of the fuel supply system. A fuel-air mixture passes through a pressurized crankcase system to the engine combustion chamber. The heavier fractions of the fuel, comprising a gasoline and oil mixture, tend to condense inside the crankcase and collect in low lying areas of the crankcase. This phenomenon is known as "puddling" and causes erratic idle and low speed operation, and when the engine speed is increased, the oil-rich fuel puddles are injected into the combustion chamber resulting in undesirable smoking and misfiring of the engine. Fuel puddling in a two-cycle engine crankcase is aggravated by cool temperatures. The fuel does not mix and vaporize as well in cool or cold air and, therefore, condenses more readily to form puddles at lower temperatures.
Cool humid weather, and the resultant intake of cool air to the carburetor, is also conducive to carburetor icing. Carburetor icing results in poor engine performance and, in severe cases, stalling and temporary complete inoperability of the engine.
The calibration of carburetors for two-cycle engines must typically take into consideration a broad range of possible intake air temperatures from about 20.degree. F. to 100.degree. (-7.degree. C. to 38.degree. C.). As a result, an intermediate compromise carburetor setting is usually made which optimizes performance at an average intermediate temperature, but sacrifices performance at the high and low ends of the range. For example, the carburetor idle speed air-fuel mixture setting often must be manually changed to accommodate the temperature change from a cool morning to a hot afternoon.
In the application of two-cycle engines to marine use, such as in outboard boat motors, emphasis in the past has more typically been on high speed performance, and performance at idle or low speeds has been, to some extent, overlooked or sacrificed. More recently, however, low speed engine performance and operation at idle speeds have drawn attention and attempts have been made to correct or alleviate some of these performance problems.
Various methods and apparatus are known in the prior art to eliminate fuel puddling or the adverse effects thereof. A simple early solution involved draining the puddles from the low lying areas within the crankcase directly to the outside. However, potential environmental concerns have lead to other solutions of the problem. U.S. Pat. Nos. 3,800,753, 805,751, and 4,590,897 describe apparatus for collecting puddled fuel in the crankcase and recycling it within the engine for eventual combustion. U.S. Pat. No. 4,534,333 describes an idle speed air-fuel passage which is heated by directing the passage through the engine exhaust chest or manifold to heat the air-fuel mixture before entry into the crankcase and combustion chamber. A similar system for heating the idle air-fuel mixture is shown in U.S. Pat. No. 2,388,331. U.S. Pat. No. 4,333,425 describes a fuel system in which heat from the exhaust manifold is used to warm a relatively small volume of idle air which is then mixed with fuel and supplied to the engine via an idle fuel-air system separate from the main fuel-air supply system. U.S. Pat. No. 4,082,068 discloses the thermostatic control of engine cooling water to maintain a high engine temperature at idle to help prevent condensation and puddling of fuel in the crankcase.
Automotive applications of four-cycle engines include the use of exhaust manifold heat to help control the temperature of induction air. However, because the fuel systems in two-cycle and four-cycle engines are substantially different in construction and operation, fuel puddling does not occur in the latter.
It has been found that, if the engine is maintained at an operating temperature of approximately 140.degree. F. (60.degree. C.) or higher, fuel puddling problems are significantly reduced because the higher temperature minimizes condensation of the heavier fuel-oil fractions. However, maintaining a higher engine block temperature only indirectly raises the temperature of the engine crankcase. More importantly, however, fuel puddling is not a problem at high engine speed and, in addition, lower engine temperatures result in better and more efficient high speed operation.
The various prior art apparatus for eliminating puddling, identified above, require substantial modifications to the carburetor and other parts of the fuel supply system and are only needed to prevent or alleviate problems occurring during the limited periods of idle or low speed operation. Therefore, it would be desirable to have a simple, yet effective, apparatus for eliminating fuel puddling and carburetor icing, as well as improving the idle speed calibration of carburetors on two-cycle engines.